A relative of the Joseph DeAngelo, the suspected Golden State Killer and East Area Rapist, claims that DeAngelo witnessed his younger sister’s rape on an air force base when he was nine years old.

Jesse Ryland, DeAngelo’s nephew, told BuzzFeedthat he learned his mother was raped at seven years old, and that Joseph witnessed the attack. Ryland said his mother, Constance, told him that she and her brother were playing in an abandoned warehouse at the German air force base where the family was living when two airman came into the warehouse and raped her with Joseph looked on.

“That’s pretty crazy for a kid to see his sister be violated,” Ryland told Buzzfeed. “Maybe that was the start of Joe going wacko.”

Ryland reportedly said that after the siblings told their parents what happened, their parents instructed them not to talk about it.

Kenneth Ryland Sr., Constance’s ex-husband and Jesse’s father, told Buzzfeed that his wife of 26 years never mentioned the rape. He said that Jesse’s sister had only recently told him about it.

Ann Wolbert Burgess, a psychiatric nursing professor who has studied dozens of serial killer cases, told Buzzfeed that if DeAngelo witnessed such an attack at a young age, it could have caused him to become preoccupied with rape.

“Of course that would be significant and could have set the nucleus of the fantasy,” Burgess said. “What probably happened was that it was something that he kept on his mind.”

DeAngelo appeared in a Sacramento courthouse on Monday for a hearing about warrants connected to the investigation. Multiple media outlets have requested that the search and arrest warrants be unsealed. According to the San Francisco Chronicle, a decision was not made on Monday, as DeAngelo’s public defender Diane Howard said she is still awaiting documents detailing the items seized from DeAngelo’s home. The hearing will be continued until May 29.